1. Field of the Invention
The present invention pertains to chemical process towers and, more particularly, to an improved catalyst assembly for enhancement of liquid-phase tower operation.
2. History of the Prior Art
Distillation columns are utilized to separate selected components from a multicomponent stream. Generally, such gas-liquid contact columns utilize either trays, packing or combinations thereof. In recent years the trend has been to replace the so-called "bubble caps" by sieve and valve trays in most tray column designs, and the popularity of packed columns, either random (dumped) or structured packing have been utilized in combination with the trays in order to effect improved separation of the components in the stream.
Successful fractionation in the column is dependent upon intimate contact between liquid and vapor phases. Some vapor and liquid contact devices, such as trays, are characterized by relatively high pressure drop and relatively high liquid hold-up. Another type of vapor and liquid contact apparatus, namely structured high-efficiency packing, has also become popular for certain applications. Such packing is energy efficient, because it has low pressure drop and low liquid hold-up. However, these very properties at times make columns equipped with structured packing difficult to operate in a stable, consistent manner. Moreover, many applications simply require the use of trays.
Fractionation column trays come in two configurations: cross-flow and counter flow. The trays generally consist of a solid tray or deck having a plurality of apertures and are installed on support rings within the tower. In cross-flow trays, vapor ascends through the apertures and contacts the liquid moving across the tray through the "active" area thereof. It is in this area liquid and vapor mix and fractionation occurs. The liquid is directed onto the tray by means of a vertical channel from the tray above. This channel is referred to as the Inlet Downcomer. The liquid moves across the tray and exits through a similar channel referred to as the Exit Downcomer. Such downcomers are located where there is a sufficient volume of liquid to effect a liquid-phase, chemical reaction, in the case of catalytic distillation. The location of the downcomers determine the flow pattern of the liquid. If there are two Inlet Downcomers and the liquid is split into two streams over each tray, it is called a two-pass tray. If there is only one Inlet and one Outlet Downcomer on opposite sides of the tray, it is called a single-pass tray. For two or more passes, the tray is often referred to as a multipass tray. The number of passes generally increases as the required (design) liquid rate increases. It is the active area of the tray, however, which is of critical concern.
Not all areas of a tray are active for vapor-liquid contact. For example, the area under the Inlet Downcomer is generally a solid region. To attempt to gain more area of the tray for vapor/liquid contact, the downcomers are often sloped. The maximum vapor/liquid handling capacity of the tray generally increases with an increase in the active or Bubbling Area. There is, however, a limit as to how far one can slope the downcomer(s) in order to increase the Bubbling Area, otherwise the channel will become too small. This can restrict the flow of the liquid and/or restrict the disengagement of vapors retained in the liquid or generated in the downcomers, cause liquid to back up in the downcomer, and thus prematurely limit the normal maximum vapor/liquid handling capacity of the tray.
A variation in design for increasing the Bubbling Area and hence vapor/liquid handling capacity is a Multiple Downcomer (MD) tray. There is usually a plurality of box shaped vertical channels installed in a symmetrical pattern across the tray to direct liquid onto and off of the tray. The downcomers do not extend all the way to the tray below but stop short of the tray by a predetermined distance which is limited by a sufficient space to permit disengagement of any vapor retained in the liquid entering the Exit Downcomer. The downcomer pattern is rotated 90 degrees between successive trays. The bottom of the boxes is solid except for slots that direct the liquid onto the Bubbling Area of the tray below, in between the outlet downcomers of said tray.
The technology of gas-liquid contact addresses many performance issues. Examples are seen in several prior art patents, which include U.S. Pat. Nos. 3,959,419, 4,604,247 and 4,597,916, each assigned to the assignee of the present invention and U.S. Pat. No. 4,603,022 issued to Mitsubishi Jukogyo Kabushiki Kaisha of Tokyo, Japan. Another reference is seen in U.S. Pat. No. 4,499,035 assigned to Union Carbide Corporation that teaches a gas-liquid contacting tray with improved inlet bubbling means. A cross-flow tray of the type described above is therein shown with improved means for initiating bubble activity at the tray inlet comprising spaced apart, imperforate wall members extending substantially vertically upwardly and transverse to the liquid flow path. The structural configuration is said to promote activity over a larger tray surface than that afforded by simple perforated tray assemblies. This is accomplished in part by providing a raised region adjacent the downcomer area for facilitating vapor ascension therethrough.
U.S. Pat. No. 4,550,000 assigned to Shell Oil Company teaches apparatus for contacting a liquid with a gas in a relationship between vertically stacked trays in a tower. The apertures in a given tray are provided for the passage of gas in a manner less hampered by liquid coming from a discharge means of the next upper tray. This is provided by perforated housings secured to the tray deck beneath the downcomers for breaking up the descending liquid flow. Such advances improve tray efficiency within the confines of prior art structures. Likewise, U.S. Pat. No. 4,543,219 assigned to Nippon Kayaku Kabushiki Kaisha of Tokyo, Japan teaches a baffle tray tower. The operational parameters of high gas liquid contact efficiency and the need for low pressure loss are set forth. Such references are useful in illustrating the need for high-efficiency vapor/liquid contact in tray process towers. U.S. Pat. No. 4,504,426 issued to Carl T. Chuang et al. and assigned to Atomic Energy of Canada Limited is yet another example of a gas-liquid contacting apparatus. This reference likewise teaches the multitude of advantages in improving efficiency in fractionation and modifications in downcomer-tray designs. The perforated area of the tray is extended beneath the downcomer with between 0 to 25% less perforation area.
A more recent use of distillation columns is for simultaneous or stagewise fractionation in conjunction with a chemical reaction. According to Le Chatelier's Principle well known in the chemical industry, the kinetics of a chemical reaction can be improved by changing the equilibrium of the reactants with the products. For example, the mechanical designs seen in U.S. Pat. Nos. 3,629,478 and 3,634,534 illustrate distillation column reactors with catalyst in the downcomers. In the case of an exothermic reaction with gaseous product being generated, the vapor disengaging space at the entrance to the downcomer could become grossly overloaded, causing the entire tower to fail in its intended distillation function. One operational consideration is thus, venting of the gases generated within the downcomer, as well as those entrained from the tray deck. Such considerations must be addressed when maximizing tower operational efficiency.
Also according to Le Chatelier's Principle, the equilibrium of a chemical reaction can be shifted from the feed components to the products by removal of the product from the reaction mixture. In many cases, the product is removed from the residual feed by subsequent distillation. The present invention allows the reaction and production separation to be accomplished in the same vessel, allowing for a more complete chemical reaction and reduced capital costs. In cases where the reaction is exothermic, the heat of reaction can be directly applied to the product recovery step. In co-pending patent application Ser. No. 08/132,059, now U.S. Pat. No. 5,389,343, assigned to the assignee of the present invention, the distillation in conjunction with chemical reaction occurs in the vapor phase. In the present invention, the reaction step occurs in the liquid phase in a discrete section of the chemical process tower.
It would be an advantage therefore to provide a method of and apparatus for enhanced distillation utilizing liquid-phase catalytic reaction. Such an assembly is provided by the present invention wherein catalyst is disposed in a liquid-phase reaction zone of the chemical process tower. Vapor flow is diverted away from this region to permit the chemical reaction to occur solely in the liquid phase. Furthermore, the liquid-phase catalytic reaction offers the process designer a different set of operating conditions to more easily circumvent the formation of certain azeotropes.